Objective: To study the process of infection and transformation of non-permissive mammalian cells by Rous sarcoma virus and to use these transformed cells for the complementation analysis of the Rous sarcoma virus (RSV) genome. Approach: A method for studying the level of RSV expression in infected mammalian cells has been developed using a quantitative cell fusion-virus rescue procedure. This method will be employed to measure virus expression in Normal Rat Kidney (NRK) cells transformed by temperature sensitive mutants of RSV. Transformed cell lines carrying different temperature sensitive mutants of RSV will be fused with the aid of Sendai virus and hybrid clones will be selected using the drug resistance markers for ouabain resistance and 6-Thioguanosine resistance. Hybrid clones will be analyzed for complementation of the RSV mutants, i.e. loss of temperature sensitive characteristics of transformation and/or virus replication as measured in a cell fusion- virus rescue assay. Recent progress: The cell fusion-virus rescue assay has been developed and characterized. Preliminary reports have been published. (Virology 62:512 (1974); Virology 62:522 (1974); Cell 3:71 (1974)).